1. Field of the Invention
This invention generally relates to broadband antennas and more particularly to antennas used in the transmission and reception of radio frequencies in the VHF spectrum.
2. Description of the Prior Art
A radio antenna is the structure associated with the region of transition between a guided wave and free space, or vice-versa. In the past, many different types of antennas have been designed both to optimize the receipt and the transmission of radio signals. The antenna of the present invention deals primarily with the transmission and reception of VHF signals, but may be used in frequency bands other than VHF as well.
Many different types of antenna configurations are well known. Among these are dipole, loop, helical, biconical, cylindrical and linear antennas, as well as reflector-type, slot, horn and complementary antennas. Other types of antennas such as long wire antennas are also well known. Of particular importance in antennas for the transmission of radio signals is the pattern of radiation emitted from the antenna. It is preferable to have the maximum radiation be in the horizontal plane. A category of antennas known as ground plane antennas is particularly well suited to providing horizontal radiation. A complete technical discussion of ground plane as well as the other types of antennas mentioned can be found in a book entitled Antennas, by John D. Kraus, Ph.D., published by the McGraw-Hill Book Company, Inc.
One particular type of ground plane antenna is known as the discone antenna. A discone antenna is constructed by fabricating a cone of material such as wire screen and placing perpendicular to the apex thereof a disc also formed of wire screen. The discone antenna functions as a wide bandwidth, impedance matching transformer, coupling a low impedance transmission line to the higher impedance of free space. In the process it radiates with a pattern similar to that of a quarter-wavelength vertical antenna above a ground plane. Waves form at the feed point (apex) and travel on the antenna surface to the edges of the cone and disc.
While conventional discone antennas function well over a wide bandwidth, for example the 144, 220 and 420 megahertz bands, they suffer the disadvantage of being rather bulky and unless carefully constructed, subject to destruction by high winds. Further, discone antennas are not easily adaptable to mobile use.
It would be highly advantageous to have an antenna which exhibits the desirable electrical characteristics of the discone antenna but which is portable and suitable for use in automobiles, trucks and other such vehicles. The antenna should be compact, lightweight, and easy to install. Ideally, it would be inexpensive to manufacture.